Proteoglycans are widely distributed, membrane-anchored glycoproteins that have covalently linked extracellular side-chains containing glycosaminoglycan (GAG) molecules such as heparan sulfate, a polymer of repeating disaccharide subunits. GAG side chains can be of different lengths and are subject to modification by sulfation and epimerization, and their structures serve as specific recognition sites for various ligands, including growth factors, extracellular matrix components, and other cell surface molecules. Heparan sulfate proteoglycans have been implicated in the regulation of numerous cellular processes, including coagulation cascades, growth factor signaling, lipase binding and activity, cell adhesion to the extracellular matrix and subsequent cytoskeletal organization, proliferation, differentiation, inflammation, microbial invasion, and tumor metastasis.
The syndecans make up a class of the heparan sulfate proteoglycans that are present on most cell types. Syndecans appear to play modulatory roles as coreceptors by presenting growth factors to their primary receptors or by increasing the infectivity of viruses by interacting with their primary receptors. See, Woods (2001) J. Clin. Invest. 107:935-941; and Elenius and Jalkanen (1994) J. Cell Sci. 107:2975-2982. Syndecans also have been implicated in neurite outgrowth, limb development, cell adhesion, and epithelial morphogenesis.